1. Field of the Invention
The present invention relates to a surface acoustic wave device having a balanced-to-unbalanced conversion function and also relates to a communication device including such a surface acoustic wave device.
2. Description of the Related Art
Technological advances in smaller sized and lighter weight communication devices such as cellular phones in recent years have been remarkable. In order to achieve these advances, not only is the number of component parts reduced and each component part is miniaturized, but also parts in which a plurality of functions are combined have been increasingly developed.
In view of such circumstances, surface acoustic wave devices used in the RF stage of a cellular phone, to which a balanced-to-unbalanced conversion function, that is, a so-called balun function, is added, have been intensively researched in recent years, and have come to be mainly used in GSM (Global System for Mobile communications) apparatuses. Several patent applications for such surface acoustic wave devices having a balanced-to-unbalanced conversion function have been filed.
Recently, in surface acoustic wave devices having a balanced-to-unbalanced conversion function, there has been a demand for increasing not only the amount of attenuation of in-phase components (common mode) within the passband, but also those outside the passband.
For example, in a surface acoustic wave device having a balanced-to-unbalanced conversion function, there is one type in which a resonator is added between a balanced signal terminal and a surface acoustic wave filter, and furthermore, the cross width and the number of electrode fingers in the resonator are different (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-317642).
However, in a surface acoustic wave device having a balanced-to-unbalanced conversion function using a longitudinally coupled resonator-type surface acoustic wave filter, there are problems in that the deterioration of the degree of balance due to the bridge capacity, etc., in particular, at higher frequencies of the passband, is severe, and the amount of common mode attenuation is small in this frequency band (or the common-mode signal level is large).